Abstract:
Urbanization alters stream hydrographs and as a result has detrimental effects on water quality, stream morphology, and stream ecosystem function. Urbanization can also change stream baseflow, although the patterns of this change has not been well-documented. A thorough understanding of this alteration is crucial for effective and sustainable water management, particularly in semi-arid regions which are urbanizing rapidly. This talk presents an analysis of the changes to streamflow in the semi-arid area of Denver, Colorado, USA with a focus on (1) contributors to streamflow baseflow and (2) an analysis of changes in streamflow response to rainfall events with urbanization.
One potential contributor to urban stream baseflow is lawn irrigation. We used water-stable isotope (δ18O and δ2H) analysis of surface water, tap water, and precipitation. Thirteen urban streams and two grassland streams were selected for sampling. An isotope mixing analysis using tap and precipitation end-members over a two week antecedent period estimated that tap water contributed 65% ± 10% to 93% ± 3% with a mean of 80% of urban baseflow on specific days in late summer. After taking contributions from infrastructure leakage into account, we estimated that lawn irrigation return flows made up 32% ± 10% to 82% ± 21% of analyzed baseflow.
For changes to streamflow response to rainfall events, we used eight years of instantaneous streamflow data in 21 watersheds ranging in size from 1 to 90 km2 with impervious areas ranging from 1% to 47%. Using a semi-automated method to identify a total of 2,877 streamflow events, we analyzed event-based metrics of peak flow, runoff depth, runoff ratio, time to peak, duration, and number of streamflow events occurring in response to rainfall events, in addition to precipitation threshold and zero flow. We found that more impervious watersheds had perennial or nearly perennial flow, unlike the least impervious watersheds which usually were dry. The more impervious watersheds also had a smaller threshold of precipitation needed to produce a response in streamflow, leading to more frequent streamflow events in response to rain. These streamflow events were shorter in duration and had higher peak flow in watersheds with more impervious surface cover. This work points to the need for local adaptation of stormwater management that seeks to mitigate the effects of streamflow changes with urbanization.
Bio:
Aditi Bhaskar is an Assistant Professor in the Department of Civil and Environmental Engineering at Colorado State University. She specializes in changes to hydrology that accompany urban development, with a focus on interactions between streams, groundwater, stormwater, and landscape irrigation. Dr. Bhaskar received a Sc.B. in Geology-Physics/Math from Brown University in Providence, Rhode Island, and a Ph.D. in Environmental Engineering from University of Maryland, Baltimore County. She was a graduate trainee of the National Science Foundation Integrative Graduate Education and Research Traineeship (IGERT) in “Water in the Urban Environment” at UMBC. Dr. Bhaskar was an NSF Earth Sciences Postdoctoral Fellow, which took her to the Eastern Geographic Science Center at the U.S. Geological Survey in Reston, Virginia, before joining CSU.
